1. Field of the Invention
This invention concerns a method of recovering products from a subterranean deposit, and more specifically to an in situ retorting method and system for recovering carbonaceous materials from a subterranean deposit.
2. Description of the Prior Art
As the world's petroleum and gas reserves are depleted, more attention is being directed to the world's oil shale deposits. Oil shale is a highly consolidated rock composed of a complex mixture of organic and inorganic constituents. The organic portion is an amorphous organic solid (called kerogen) which will decompose or pyrolyze when heated to temperatures above 500.degree. F to provide fluid hydrocarbons commonly termed "shale oil."
Considerable research has been conducted to develop economic methods of recovering hydrocarbon products from oil shale deposits. Methods suggested can generally be divided into two categories: surface retorting and in situ retorting.
Surface retorting involves mining the oil shale, transporting it to the surface, crushing the shale, and then forcing it through a surface retort to extract the recoverable hydrocarbon products. Although surface retorting processes have been investigated for many years, problems inherent in this process has deterred widespread commercial application. Typically, mining is expensive and there are environmental problems associated with removing the shale and with disposing the spent shale.
In situ retorting processes involve heating the shale in situ to pyrolization temperature either by in situ combustion or by passing externally heated gas through the shale and removing the gas and liquid products to the surface through shafts or wells. In situ processes may significantly reduce environmental problems such as surface disfigurement caused by surface mining and the need for disposing spent oil shale from surface retorts. It also may be more economic than mining in deeper deposits and in lower grade oil shale.
A prerequisite to in situ retorting is creating adequate permeability in the shale deposit to provide passages for the retorting fluid, good heat transfer to the shale, and paths for the retorted values. Since oil shale deposits typically do not have sufficient permeability to carry out in situ retorting processes, methods have been proposed to create this permeability. One such method is explosive, hydraulic, or electrical fracturing. Fracturing, however, is generally not as economic and efficient as certain other fragmenting techniques since it is generally difficult to fragment the entire shale deposit.
Another method of increasing oil shale permeability is to use nuclear explosions to create a rubble-filled chimney. A nuclear blast produces a cavity mainly by displacing the surface upward. As the rock condenses to a liquid and begins to collect at the bottom of the cavity the decrease in pressure causes the roof to collapse. Shale falls into the cavity creating a column of broken oil shale, generally called a rubble pile. Unfortunately, this technique is applicable only to deep formations and there are potential problems associated with radioactivity and damage and inconvenience to residents caused by ground motion.
A more recent method of increasing shale permeability is a modified in situ technique in which a portion of the oil shale at the base of the oil shale is excavated to create a void space. The remaining exposed shale is then allowed to collapse by itself or with the aid of explosives. In either case, the deposit to be retorted is expanded into a larger volume than originally occupied. The resulting rubble pile is then retorted. An example of this method is described in U.S. Pat. No. 3,661,423, issued May 9, 1972, to Garrett. Unfortunately, these "mining and collapse" methods also suffer drawbacks. One difficulty is that the shale oil to be mined may not be rich enough to justify mining and surface treatment. Another problem is that it is often difficult to mine oil shale and special precautions are often necessary to ensure safety of the miners. Still another problem is that substantial amounts of shale are left undisturbed in order to form walls which define and separate the rubble piles. Due to the relatively impermeable nature of oil shale, only a small portion of these solid walls will be retorted, therefore, significant portions of the hydrocarbons in these walls may not be recovered.